Radio connections must achieve a certain correctness so that they can transmit information in a desired manner. This can be achieved with a sufficiently high C/I ratio (Carrier to Interference Ratio), which represents the ratio of the received carrier power to the simultaneously received interference power. For prior art cellular systems it is typical that a certain target level is defined for the C/I ratio (or for the SIR—Signal to Interference Ratio—of for the S/N—Signal to Noise ratio—or for the S/(I+N)—Signal to Noise plus Interference ratio—or for some other corresponding factor), and that the transmit power for each radio connection is controlled to be so high that the target level is barely reached. It is not beneficial to have a higher transmit power than that which is required to reach the target level of the C/I ratio, because an unnecessary high transmission power consumes electric energy in the transmitting device and causes interference to other simultaneous radio connections.
In the CDMA system the SIR value of the i:th packet of a cell can be calculated with the formula below:
                              SIR          i                =                              G            i                    ⁢                                    P                              rx                ,                i                                                                                      ∑                  j                                ⁢                                                                  ⁢                                  P                                      rx                    ,                    j                                                              +                              P                other                            +                              P                N                                                                        (        1        )            where Prx,i is the power received by the i:th user,
      ∑    j    ⁢          ⁢      P          rx      ,      j      is the total power of the own cell, Gi is the process gain of the i:th packet, Pother is the interference power of the other cells, and PN is the external temperature noise or background noise.
When a new transmission is initiated the base station must define the required transmit power in some way. If the transmit power is too low there will occur too much errors in the connection. On the other hand, if the transmit power is too high, this will interfere with the other connections in the cell. When the new transmission is initiated the required output power is typically controlled in a so called open-loop power control on the basis of link gain measurements. The output power is determined on the basis of the desired error level e.g. according to the formula (1), by using the signal level of the base station's pilot signal detected by the mobile station when value of the term Prx,i is evaluated.
FIG. 1 illustrates a situation which occurs at the beginning of a new transmission. In this schematic example there are two mobile stations within the base station area, whereby the transmit powers of their transmissions are represented by the curves A and B. In FIG. 1 a new frame begins at moment t1. At the beginning of a new frame also the mobile station C begins to transmit. The transmit power p1 used by the mobile station C at the beginning of the frame is typically determined according to the formula (1). However, the transmission C interferes with the other connections in the cell, whereby the error level of the connections will increase. Therefore the base station must control the power levels of the other transmissions, which results in that also the power level of the transmission C must be changed. The base station adjusts the transmit powers of the mobile stations until the number of errors occurring in different connections is reduced to the target level and the SIR target levels for each connection are obtained. In FIG. 1 this transmit power control is represented by the segments of the curves A, B and C between the moments t1 and t2. At the moment t2 the desired target error level is obtained, whereby the transmit power of the transmission C has changed from power p1 to power p2. However, a practice like this has an disadvantage in that the transmit powers are not optimal between the moments t1 and t2, and errors occur in the connections. A fast power control corrects the transmit powers in a desired way to their optimal values, but communication capacity is lost before the desired error levels are obtained. Regarding FIG. 1 it must be noted that for the sake of clarity it shows only the search of power levels which compensate for the interference, and that the effect of a possible fast fading is not shown.
Situations in which the power levels of the transmissions are not optimal are not only caused by the beginning of a transmission, but also by the end of a transmission, or in more general terms by a changed number of transmissions. When a new frame begins the powers used for transmissions which extend over the frame boundary, such as the transmissions A and B in the example of FIG. 1, are based on the interference situations of the previous frame or of previous frames. Thus the power levels of the transmissions A and B are calculated on the basis of interfering transmissions which are different from those being active in the new frame. For instance half of the packets in the considered frame can relate to active bearers in the previous frame and half of the packets can relate to non-active bearers in the previous frame. In such an example half of the transmissions causing interference in the previous frame are not anymore active during the new frame, whereby the power levels used at the end of the previous frame are not anymore correct at the beginning of the new frame.
The term “bearer” means an entity formed by all such factors which have an impact on the communication between the base station and a certain terminal. The term bearer relates i.a. to the transmission rate, delay, bit error rate, and variations of these between certain minimum and maximum values. A bearer can be perceived as a communication channel formed by the added effect of these factors, whereby the channel connects the base station and a certain terminal and can carry useful data, i.e. payload information. One bearer always connects only one terminal to one base station. Multimode terminals can simultaneously maintain several bearers, which connect the terminal to one base station. If the system is able to use macro diversity combination, then the bearer or bearers can simultaneously connect the terminal to the network via more than one base station.
The problem described in the previous paragraphs is particularly inconvenient in packet traffic. If the connections between the base station and the mobile stations are so called real time connections (RT connections), such as for instance voice connections, then the transmissions typically extend as uniform transmissions over several frames, whereby the search for the power level at the beginning of the transmission represents a very small proportion of the transmission time. In packet communication, more generally in non-real time connections (NRT connections), the data is transmitted in a minimum case as packets with the length of one frame, whereby the search for the power level at the beginning of the packet transmission forms a considerable proportion of the transmission time.
One way to solve the above described problems is to recalculate the power levels of all transmissions with the open loop principle when the number of transmissions changes. However, such a solution has a disadvantage in that it does not consider the effect of the fast power control on the transmit power of bearers, which were active in the previous frame. When required the base station continuously adjusts the output powers. The base station must adjust the output powers, typically to compensate for the effects of the slow fading and the fast fading on different connections and for the effect of mutual interference between the different transmissions.
For instance, if the base station during a frame can give at most 16 power control instructions, with which the transmit power increases or decreases 1 dB, then due to a momentary change in the conditions the transmit power of one transmission can rise 16 dB during the same frame, or be multiplied by 40, or decrease to 1/40 compared to the output power at the beginning of the frame. If the output power calculated with the open loop principle deviates less than 16 dB from the correct value, then the power control has time to adjust the output power to the correct value during one frame. However, the values calculated with the open loop principle can deviate much more from the correct value, whereby the power control does not have time to correct the power to the correct value during one frame.